As a press plate for offset printing, not only a presensitized plate using an aluminum plate as a substrate (hereinafter referred to as "a PS plate") but also a paper plate using a paper substrate and a film plate using a plastic film as a substrate have been prevailingly used. In particular, the use of a low-priced paper or film plate has markedly increased in frequency because of a recent tendency to diversify the purpose of printing and require a small number of copies in printing. Under these circumstances, reduction in time required for replacement work of machine plates has become a great problem.
For instance, in a case where the printing of a small number of copies, such as 300 copies, was performed in the past, the time taken from the start to the finish of printing was about 5 minutes, while the replacement work of machine plates took at least 5 minutes. In other words, it occurred in some cases that the time for attaching a machine plate was longer than a net printing time. Therefore, many devices which enable reduction in working time for replacement of machine plates have been proposed in recent years. For instance, such devices are disclosed in Tokkai Sho 59-1262, Tokkai Hei 3-93546, Tokkai Hei 6-328669, Tokkai Hei 8-39779 (The term "Tokkai" as used herein means an "unexamined published Japanese patent application"), Jikkai Hei 3-19037, Jikkai Hei 3-45133, Jikkai Hei 3-45134 (The term "Jikkai" as used herein means an "unexamined published Japanese utility model application"), and Jikko Hei 6-11782 (The term "Jikko" as used herein means an "examined Japanese utility model publication").
Each of those devices requires a plate cylinder or/and a plate lock-up device to undergo certain direct processing. However, it is difficult to perform accurate processing at the job site. In attaching such a device to a printing machine which has already been in operation, therefore, it is necessary to detach the parts to be worked from the printing machine and process them with a NC machine in order to secure printing accuracy; as a result, the operations at the job site increases in complexity.